The present invention relates to a process to prepare aqueous dispersions of anionic polyurethanes and polyurethane-polyacrylate hybrids containing tertiary amino functional acrylic oligomers, polymers or copolymers as acid-neutralizing agents, in the absence of volatile tertiary amines.
Aqueous dispersions of polyurethanes or polyurethane-polyacrylate hybrids are well known as basis for the production of coating compositions. They may be used for protective or decorative coating, optionally in combination with additives like coloring agents, pigments, matting agents, and the like. Polyurethanes can possess many desirable properties such as good chemical resistance, water resistance, solvent resistance, toughness, abrasion resistance, durality. Thermoplastic polyurethane-polyacrylate hybrid dispersions became of interest regarding to polyurethanes because of their lower raw material costs and they became of interest regarding to polyacrylates because of their better performance.
As is by now well known in the art, aqueous polyurethane dispersions are particularly advantageously prepared by dispersing an isocyanate-terminated polyurethane prepolymer bearing ionic and/or non-ionic dispersing groups into an aqueous medium and than reacting the prepolymer with an active hydrogen containing chain extender while dispersed in the aqueous medium. See e.g. UK-patents 1549458 and 1549459.
Generally polyurethane-polyacrylate hybrids are prepared by an addition polymerisation of acrylic monomers in a polyurethane dispersion. The acrylic monomers can be added to the polyurethane dispersion, which is described in for example DE 1953348, EP643734 but they can also be added during the preparation of the polyurethane dispersion at several stages, which is described in for example U.S. Pat. No. 4,644,030, EP742239. The vinylic monomers can be added during or after the polyurethane prepolymer formation and in these cases they function as viscosity reducing solvents. The advantage is that no or less other organic solvents have to be used, and a better homogeneity is obtained. The addition polymerisation is executed after the formation of the aqueous polyurethane. Moreover, further vinylic monomers may be added during the polymerisation as described in for example EP 308115.
Dispersibility of the polyurethanes or polyurethane-polyacrylate hybrids in water can be achieved by incorporation of appropriate chain pendant ionic groups, chain pendant non-ionic hydrophilic groups, or in-chain non-ionic hydrophilic groups in the structure of the polyurethane polymer. If suitable, external surfactants can be applied in addition. Preferably anionic groups are incorporated into the polyurethane backbone, such as carboxylic, sulfonic, sulfate or phosphate groups, by reaction of an isocyanate reactive compound having at least one acid group with a polyisocyanate. Most common is the incorporation of a carboxylic acid functional compound.
The carboxylic acid functions are generally neutralized before or during dispersion of the polyurethane prepolymer or prepolymer-vinylic monomer solution in water with a volatile tertiary amine. Anorganic bases are less convenient, since the polyurethane will coagulate when they are applied or it will provide highly water sensitive films or coatings. To prevent coagulation it is suitable to incorporate a great number of hydrophilic polyethoxy chains into the polymer system.
A disadvantage of the application of volatile tertiary amines as neutralizing agent is that they evaporate during the film formation, and therefore will cause environmental pollution,
The present invention offers a process to prepare aqueous dispersions of anionic polyurethanes or of polyurethane-polyacrylate hybrids containing no volatile tertiairy amines.
The object of the present invention is to provide a process to prepare a dispersion of a polyurethane or a polyurethane-polyacrylate hybride which contains no volatile tertiary amines as neutralizing agents for carboxylic acid groups.
Accordingly the present invention relates to a process for the preparation of an aqueous dispersion of an anionic polyurethane in which initially a tertiary amino-functional acrylic monomer of formula I 
wherein,
R, R1, R2, R3, R4 and R5 are organic groups which have no reactivity towards the double bond or the tertiary amine function, is used as neutralizing agent for pendant carboxylic acid groups in dispersions of a polyurethane or a polyurethane/polyacrylate, whereafter the unsaturated monomers undergo in situ an addition polymerisation, optionally together with other unsaturated monomers,
characterized in that the aqueous dispersion of the anionic polyurethane is prepared by the steps of
preparation of an isocyanate functional anionic polyurethane prepolymer, optionally in the presence of vinylic monomers
mixing of the isocyanate terminated anionic polyurethane prepolymer with a tertiary-amino functional unsaturated monomer and optionally other vinylic monomers
followed by dispersion of the obtained mixture into water, and chain extension of the polyurethane prepolymer with an active hydrogen compound during or after the dispersion in water,
initiating radical polymerisation of the vinylic monomers, including the tertiary amino functional unsaturated monomers.
Advantage of this process is that after the polymerisation of the unsaturated monomers containing a tertiary amine the dispersions contain no volatile amines and therefore environmental problems will be eliminated. Another advantage of the process is that the unsaturated monomers containing a tertiairy amine function act as viscosity reducing solvents during the formation of the aqueous polyurethane. This effect is improved when other unsaturated monomers are present as well and an aqueous dispersion of a polyurethane-polyacrylate hybride is formed. In both situations less or sometimes even no other organic solvents have to be be used, and a better homogeneity is obtained.
A further part of the invention is a process in which the isocyanate terminated prepolymer is reacted with 0-100% of a stoichiometric amount of a hydroxy functional unsaturated monomer before the dispersion in water.
As a result the polyurethane prepolymer is completely or partially functionalized with acrylic double bonds by complete or partial reaction of the isocyanate functions of the prepolymer with a hydroxy functional unsaturated monomer. As a result the polyurethane polymer system will contain double bonds and will contribute to the addition polymerisation.
The acid groups in the prepolymer are initially neutralized by a tertiary amine functional unsaturated monomer. In the process of the invention a tertiary amine functional acrylic polymer is formed during the process by radical polymerisation of the tertiary-amino functional unsaturated monomers. When other unsaturated monomers are present a copolymer will be formed during the process including the tertiary amine functional unsaturated monomers and the other unsaturated monomers. The other vinylic monomers which may be present are selected from acrylic alkyl esters, methacrylic alkyl esters, styrene esters or ethers of vinyl alcohol. The other vinylic monomers are present in an amount of 0 to 90%.
The present invention further comprises the dispersions prepared by the process and coatings or films derived from dispersions prepared by the process.
At low levels of other vinylic monomers or, without the presence of vinylic monomers, the performance of the films or coatings of the dispersions will be comparable with the performance of the films or coatings of polyurethane dispersions. At higher levels of other vinylic monomers the performance of the films or coatings of the dispersions will be comparable with the performance of the films or coatings of polyurethane-polyacrylate hybride dispersions.
Both the polyurethane and the acrylic monomers may contain additional functional groups with the objective to improve the waterdispersibility, to improve adhesion to substrates at application, for performance reasons, or as potential sites for crosslinking. Suitable functions are polyalkoxy functions with a large concentration of ethoxy functions, are tertiairy amine or quaternairy amine functions, perfluor functions, incorporated silicon functions, hydrazide functions or hydrazone functions, ketone, acetoacetate, hydroxy, methylol, amide, glycidyl, ureido or aldehyde functions.
The tertiary amine functional unsaturated monomer which is used in the process of the invention can be a dialkylaminoalkyl acrylate, a dialkylaminoalkyl methacrylate, a dialkyl aminoalkoxy acrylate and/or a dialkylaminoalkoxy methacrylate. Suitable examples are dimethylaminoethyl acrylate, dimethyl-aminoethyl methacrylate, diethylaminoethyl acrylate, diethyl-aminoethyl methacrylate, 2-(diethylamino)ethanol vinylether and the like.
The tertiary amine functional unsaturated monomers are present in a ratio to the anionic residues to be neutralized in the polyurethane prepolymer from 0.3 to 2 and preferably from 0.7 to 1.5
When the isocyanate functional polyurethane prepolymer is reacted with a hydroxy functional unsaturated monomer the reaction is carried out by techniques well known in the art at 40 to 130xc2x0 C. The hydroxy functional unsaturated monomer may be hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxy butyl methacrylate, hydroxypolyester acrylates or methacrylates.
The polyurethane and/or polyurethane-polyacrylate hybride contains anionic groups to obtain water dispersibility. This anionic group can be a carboxyl, a sulfonic, a sulfate and/or a phosphate group and is preferably a carboxyl group.
The isocyanate functional polyurethane prepolymer containing carboxylic acid groups is prepared in a conventional way. EP 308115 presents an overview of the process and of suitable polyols and polyisocyanates as starting materials for such polyurethane prepolymers. Ketone functional polyester polyols are included as well. Suitable hydrogen reactive chain extenders and the conditions wherein they are used are described in the same application.
The amount of carboxylic acid functions in the isocyanate functional polyurethane prepolymer is from 1 to 15% and preferably from 2 to 10%. The carboxylic acid functions are introduced in the prepolymer by incorporation of a dihydroxy alkanoic acid, which may be a 2,2-dimethylol alkanoic acid and is preferably 2,2-dimethylol propanoic acid.
In order to obtain polyurethane-polyacrylate hybrids, other vinylic monomers may be added. They can be added completely or partially to the polyurethane prepolymer or, preferably the polyurethane prepolymer is formed in the presence of the vinylic monomers. By combining the polyurethane prepolymer with the vinylic monomers an optimal homogenity is obtained and the viscosity will be reduced. As a consequence less additional solvents will be needed. The vinylic monomers can be further added completely or partially after dispersion of the polyurethane prepolymer in water, during or after the chain extension. They further can be partially added during the radical polymerisation process.
Suitable vinylic monomers which can be used in addition are selected from acrylic or methacrylic alkyl esters, acrylic or methacrylic alkyl esters, optionally functionalized with hydroxy, quaternary amines or halogen groups, acrylonitrile, styrene, esters and ethers of vinyl alcohol.
The final proportion of additional vinylic monomers may vary from 0 up to 90%, and preferably from 0 to 60%.
The tertiary amine functional unsaturated monomer may be mixed with the polyurethane prepolymer, optionally in the presence of other vinylic monomers and be dispersed in water using techniques well known in the art. Preferably, water is stirred into the mixture with agitation or, alternatively, the mixture is added to the water and optionally the chain extender with agitation. Alternatively the polyurethane prepolymer, optionally in the presence of other vinylic monomers is added to a mixture of water and the tertiary amine functional unsaturated monomer and optionally the chain extender with agitation.
Regularly the ratio of tertiairy amine functions to the acid groups in the polyurethane prepolymer is from 0.3 to 2 and preferably from 0.7 to 1.5. At lower levels of the tertiary amine functional acrylate, the final dispersion will have a lower pH value, which may be advantageous for some crosslinking applications.
Polymerisation of the tertiary amine functional unsaturated compound or combination of the tertiary amine functional unsaturated compound and other vinylic monomers may be effected by the methods described in EP308115.
The polyurethane and/or the acrylic monomers of the invention may contain additional functional groups which may be polyalkoxy functions with a large concentration of ethoxy functions, may be tertiary amine or quaternary amine functions, perfluor functions, incorporated silicon functions, hydrazide functions or hydrazone functions, ketone, aceto-acetate, hydroxy, methylol, amide, glycidyl, ureido or aldehyde functions.
Conventional non-ionic, anionic or cationic surfactants may be applied to optimize dispersion of the polymer system in water and stabilisation of the final polymer dispersion. Suitable examples may be based on long-chain dialkyl sodium sulphosuccinate, arylalkylpoly-ethoxyalkyl derivatives, highly ethoxylated polyurethane derivatives and the like.
The aqueous polymer dispersions produced by the method of the invention are stable for long periods of time. If desired minor amounts of solvents may be included in the dispersions.
Many additional ingredients may also be present in the application stage, for example fillers, colorants, pigments, silicons, flow agents, foam agents, fire retardants and the like.
The aqueous polymer dispersions produced by the method of the invention may be used in adhesives, sealants, printing ink and in coatings. They may be applied on any substrates, including leather or artificial leather, metals, wood, glass, plastics, paper, paper board, textile, non-woven, cloth, foam and the like by conventional methods, including spraying, flow-coating, roller-coating, brushing, dipping, spreading and the like.
Various aspects of the present invention are illustrated by the following examples. These examples are only illustrative and are not limiting the invention as claimed hereafter.